The present invention is directed to a method and system for imaging selected features of an object. In particular, the present invention is directed to collecting three-dimensional data concerning features of an object, and using such data to determine, for example, the dimensions and relative positions of the features.
The complexity of semiconductor chips has increased dramatically over the past several years. Such increased complexity has lead to an increase in the number of input and output leads or contacts required for each chip package. Further, with this increased complexity and the constant need to shorten chip production times, methods and systems for more rapid and accurate inspection of chip packages are needed.
Three dimensional laser beam sensor systems utilizing laser optical triangulation have been used to inspect chip packages. Such a system consists of a semiconductor diode laser, a beam deflector (for example, an acousto-optical (AO) deflector, also called an AO modulator) and a position sensitive device (PSD). The laser diode provides the light source for measurements. The beam deflector directs the laser beam to xe2x80x9csweepxe2x80x9d the target object along a xe2x80x9csweepxe2x80x9d direction. It will be observed that the terms xe2x80x9csweepxe2x80x9d and xe2x80x9cscanxe2x80x9d may be used interchangeably. However, xe2x80x9csweepxe2x80x9d will be used generally herein to refer to the specific xe2x80x9cscanningxe2x80x9d produced by a beam deflector, or AO deflector. Accordingly, to cover the entire target area, such systems typically rely on a mechanical scan, or translation, of the sensor system, or equivalently, the target object, in a direction perpendicular to the AO sweep direction. The PSD measures the height of the target object at each scan point and the data are stored until an image record for the entire object or a selected portion is collected. The stored image record may then be compared to a manufacturer""s specification for the object or a selected portion to determine whether the object meets specification.
U.S. Pat. No. 5,554,858 issued to Costa et al. (the xe2x80x9c""858 patentxe2x80x9d), expressly incorporated herein by reference, describes one such system. A laser light source combined with an AO deflector is positioned to illuminate an object and sweep along the AO deflection direction while commercially available linear motion tables provide the transverse scanning translation. PSD sensors are positioned on both sides of the incident beam to receive light reflected from the sample and imaged into the PSDs by lenses. Further, the ""858 patent describes use of multi-channel PSDs to collect the imaging data. A PSD provides an analog output current ratio proportional to the position of a light spot falling along its length. A multi-channel PSD has a segmented photo-sensitive area, the multiple segments comprising the multiple data channels. When used with a selective sampling technique, the multi-channel PSD can reduce the effect of stray or multipli-reflected light.
U.S. Pat. application Ser. No. 08/680,342 to Liu et al. (the xe2x80x9c""342 applicationxe2x80x9d), expressly incorporated herein by reference, describes another such system. The ""342 application describes another scanning system using optical triangulation techniques. This system uses a laser beam and AO deflector and one or more photosensitive devices to collect light reflected off-axially (from the axis of the incident light source) from the target object. Further, the system uses a polarizing beam splitter in the optical path of the incident beam to direct light reflected co-axially from the target object into a photo diode array for intensity measurement.
In conventional three-dimensional scanning systems such as the aforementioned, the AO deflecting swath is limited by the finite bandwidth of the AO device. An AO device deflects an optical beam by having an ultrasonic sound wave applied by piezo-electric signal transducers to a suitable crystalline material. The resultant sound wave in the crystal produces a periodic variation in the crystal""s index of refraction which is used to diffract a fraction of an incident beam of monochromatic light. This fraction of the incident beam of monochromatic light is called the first order diffracted output. If no RF signal is applied, only the zero order non-diffracted incident beam will exit from the AO device.
The angle of diffraction xcex8 of the first order output from the incident beam direction is given approximately by:                     θ        =                              λ            ⁢                          xe2x80x83                        ⁢            f                    V                                    (        1        )            
where xcex is the wavelength of the incident beam measured in air, f is the ultrasonic frequency of the sound wave, and V is the ultrasonic velocity of the sound wave. Those skilled in the art will appreciate from equation (1) that the diffraction angle xcex8 varies directly with the ultrasonic frequency f. Therefore, since the process is linear, an AO diffraction angle range or xe2x80x9csweep anglexe2x80x9d range, xcex94xcex8, is proportional to a change in the ultrasonic frequency, xcex94f, according to:                               Δ          ⁢                      xe2x80x83                    ⁢          θ                =                              λ            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            f                    V                                    (        2        )            
The first order output can be quickly xe2x80x9csweptxe2x80x9d through the AO sweep angle range by continuously changing the ultrasonic drive frequency to the AO deflector. Equation (2) shows that the maximum AO sweep angle range is limited by the bandwidth, or range of frequencies, which can be used with the AO deflector. Thus, the smaller the AO bandwidth, the smaller the concomitant AO sweep angle range and the more mechanical scans required to complete a given area measurement. For example, to cover a 12xe2x80x3xc3x978xe2x80x3 ball grid array tray or an 8xe2x80x3 flip chip wafer with an AO deflector scanning with a single beam, several hundred parallel mechanical scans might be required.
Other beam deflectors such as rotating polygonal scanners or galvanometer mirrors can also sweep a beam through a finite angle in a finite time. The speed limitations of these devices similarly impose a speed constraint on the operation of any scanning system in which they function.
Further, the speed of a system using a conventional PSD will be limited by the design and physical parameters of the PSD and its associated electronics. A current state-of-the-art PSD may have rise and fall times of a few hundred nanoseconds each. Therefore, the maximum throughput from a real time 3D sensor is limited by the bandwidths of both the AO deflector and the PSD.
The present invention is directed to a system for quickly and accurately measuring features of objects. In an exemplary embodiment, a system is provided that simultaneously scans with and collects data from multiple laser beams incident upon the target object. Specifically, multiple laser beams are swept across a sample by a beam deflector and the multiple reflected beams are imaged into one or more PSDs for determination of a 3D profile of the target object.
In accordance with the exemplary embodiment of the present invention, the light source is first transmitted through a beam deflector. The thus deflected beam is then directed to a diffracting beam splitter (DBS), also known in the art as a diffraction grating. The DBS is designed to diffract the beam into multiple orders. The resulting multiple beams are focused upon the target object to form multiple spots at the points of impingement at known X-Y positions on the object. The multiple laser beams are reflected by the object and this reflected light is imaged onto one or more multi-channel PSDs.
If an AO deflector is used with a DBS, for example, and the devices are chosen such that the separation of adjacent diffraction orders of the DBS is equal to or slightly smaller than the AO sweep angle range, xcex94xcex8, then there will be continuous angular coverage by the laser scan. The overall angular coverage of the combined devices will be xe2x80x9cnxe2x80x9d times xcex94xcex8, where xe2x80x9cnxe2x80x9d is the total number of diffraction orders produced by the DBS including the zeroth order. A typical DBS is fabricated by a lithography technique with a computer generated mask. A commercial DBS may have from a few up to several tens of diffracting orders with a dot pattern uniform in both space and energy distribution. Therefore, the present exemplary embodiment of the invention provides multiple evenly separated laser spots scanning synchronously across the target.
Analog signals generated by the one or more PSDs are related to the position(s) where the reflected light is focused on to the PSDs. These signals are used to calculate the Z coordinates of the object (i.e., the height) at the points of impingement of the source laser beams on the object. These coordinates are calculated using standard optical triangulation principles. The three-dimensional (3D) information can then be compared to manufacturer""s specifications for a packaged device to determine if, for example, each solder ball or bump on a device such as a ball grid array, is the correct height. Additionally, the information can be used for determining co-planarity of the ball tops and/or warpage of the substrate.